1. Field of the Invention
The present invention is broadly concerned with an improved method of growing or culturing acid-producing microorganisms used in cheese making. More particularly, it is concerned with an improved method which involves initially permitting an inoculated medium to incubate until the pH of the medium drops to an appropriate level, followed by raising of the pH and further incubation to completion. The method has been shown to give enhanced results, particularly in conjunction with a new lecithin-containing starter medium.
2. Description of the Prior Art
In the manufacture of natural cheese, milk in a cheese vat is inoculated with a minor amount (e.g., 2-4 percent) of a bulk starter providing the necessary culture of acid-forming microorganisms used for the particular cheese being manufactured. For example, in the case of Italian cheeses such as mozzarella, it is the usual practice to employ Streptococcus thermophilus together with one or more lactobacilli such as Lactobacillus bulgaris. In the art, the streptococci are generally referred by the short name of "coccus", while the lactobacilli are referred to as "rod" bacteria because of their appearance under microscopic examination.
The quantity and acitivity of cheese-making microorganisms can be critical to the overall outcome of the process and final cheese quality. Again referring to the Italian cheese, it has been found that, in order to make acceptable cheese, the ratio of coccus to rod organisms in the starters should be from about 1:1 to 5:1, the most preferable level being about 4:1. If these ratio considerations are not met, the final cheese product may be deficient in flavor or physical properties such as elasticity and "stringiness."
It is the universal practice among cheese makers to grow their bulk starters using relatively minor amounts of seed culture. In such techniques, the seed culture is inoculated into a starter medium, and allowed to incubate therein so that the culture cells will multiply to produce the desired bulk starter for use in cheese making. Here again, the types of starter media and the techniques used during the incubation process can have a relatively critical outcome on the quality of the final bulk starter, and hence on the cheese ultimately produced. A dilute dispersion of nonfat milk (e.g., 12 percent solids level) in water has long been considered the starter medium of choice. However, use of nonfat milk in this context is a relatively expensive proposition, and therefore cheese makers have in the past sought to use media of a less expensive nature which either eliminate nonfat milk entirely, or sharply limit its use by provision of substitute materials. Many of these proposed media include constituents such as whey or the like. Exemplary patents disclosing prior starter media include U.S. Pat. Nos. 3,998,700, 2,805,950, and 3,852,158.
The longstanding technique of starter incubation used by cheese makers has been to simply inoculate the medium (which is typically at or around neutral pH), while maintaining the medium in a heated condition (e.g., 102 degrees F.). During the incubation process the seed culture multiplies and produces acid; this in turn serves to drop the pH level of the medium down to a level of 4.0-5.0, at which time the titratable acidity of the medium is typically at an appropriate level and the incubation is then terminated by cooling to 40-50 degrees Fahrenheit. While this is the customary approach, workers in the art have devised a number of different operational methods which can, in certain circumstances, produce more or better quality cheese-making microorganisms. For example, researchers at the Utah State University have developed a lactic culture system which involves continuous neutralization of the starter medium during incubation. Specifically, use of this system involves a pH control system including pH probes, and means for injecting a base such as ammonia into the starter medium. Generally speaking, the starting pH of the medium is around 6.3, and, as the pH drops during incubation to a level of about 6.0, base is injected in order to bring the pH of the system back up to the desired 6.3 level. Accordingly, the pH of the medium using this technique is constantly maintained between 6.0 and 6.3, and is never allowed to decrease to the levels of acidity reached in traditional processes. While this approach (sometimes referred to in the art as "external pH control") has achieved substantial usage in the cheese making art, a number of problems remain. First, while the 6.0-6.3 pH range is sometimes preferable from the standpoint of bacterial growth, continual pH maintenance within this range can upset typical enzymatic systems and, in the case of Italian cheeses, the coccus/rod ratio ultimately obtained may be adversely affected. Moreover, in the traditional approach, the drop in pH to the 4.0-5.0 level has the effect to retarding the growth of pathogens; however, the external pH control system never permits the pH level to drop to this level, and accordingly pathogens which would otherwise be inactivated remain viable in the medium.
Another method developed in recent years is described in U.S. Pat. No. 4,282,255. This patent relates to a method for growing acid-producing bacteria wherein use is made of temporarily water insoluble neutralizing agents which are placed directly in the starter tank. These tablets or bodies include basic materials, and are designed to slowly and continuously release base in order to continuously maintain the pH level, typically between 5 and 7. In practice, and as disclosed in the referenced patent, these slow-release bodies require continual stirring or agitation of the starter tank. Such agitation has been found to present problems, inasmuch as it can interfere with proper cell growth and multiplication. Also, the slow-release pH control system disclosed in U.S. Pat. No. 4,282,255 (sometimes referred to in the art as "internal pH control"), suffers from the fact that the pH level is never allowed to decrease to a point where pathogens are completely killed.
In short, both the external and internal pH control systems involve an attempt to control pH within a relatively narrow band. Accordingly, a graph of pH versus time for these processes shows an initial pH drop to the desired range of pH control, followed by a generally horizontal graphical pattern. In the case of the external pH control, the graph typically is in the form of a "sawtooth" by virtue of periodic addition of base; on the other hand, the internal pH control typically generates a straighter graphical line in this region, by virtue of the slow, continual release of base in the starter media and consequent continual neutralization of acid as produced by the microorganisms.